Extraction from large thermal storage systems using phase change materials and latent heat exchangers

ABSTRACT

An energy storage method and apparatus for extraction from large thermal storage systems using phase change materials and latent heat exchangers. This includes thermal heat extraction from, and charging of a large thermal storage tank containing thousands of megawatt hours of thermal energy, using the phase change of heat collection fluid and the phase change of molten phase change material for thermal storage use in generating electricity, steam, or for other industrial processes as implemented in the field of solar energy collection, thermal storage and extraction. The method and apparatus continuously removes thermal resistance that comes from the phase change material allowing operation at a high rate of efficiency. A heat exchanger is provided inside the storage tank thereby reducing heat losses, capital costs and space requirements compared to existing thermal storage systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Patent Application Ser. No. 61/581,306 filed on 29 Dec. 2011, U.S. Patent Application No. 61/673,860 filed on 20 Jul. 2012, and U.S. Patent Application No. 61/673,861 filed on 20 Jul. 2012.

FIELD OF THE INVENTION

The present invention relates generally to energy storage. More particularly, the present invention relates to thermal heat extraction from, and charging of, a large thermal storage tank (LTST) containing thousands of megawatt hours of thermal energy, using the phase change of heat collection fluid (HCF) and the phase change of molten phase change material (PCM) for thermal storage use in generating electricity, steam, or for other industrial processes as implemented in the field of solar energy collection, thermal storage and extraction.

BACKGROUND OF THE INVENTION

Energy systems in terms of geothermal and solar heating are well known in the thermal energy arts. In terms of geothermal heating systems, wells in the Earth have been used to facilitate transfer of heat from the ground to a usable energy system. Likewise, solar heating systems utilize the sun's heat to facilitate transfer of heat from the sun's rays to a usable energy system.

Two recent examples of geothermal systems are illustrated by US Patent Application Publication No. 2009/0320475, published 31 Dec. 2009 to Parrella and US Patent Application Publication No. 2010/0276115, published 04 Nov. 2010 to Parrella. Each of these systems relate to either wells drilled specifically to produce heat or wells that have been drilled for oil and gas exploration that are either depleted, or have never produced oil or gas, usually remain abandoned and/or unused and may eventually be filled.

Numerous other known geothermal heat/electrical methods and systems for using the geothermal heat/energy from deep within a well (in order to produce a heated fluid (liquid or gas) and generate electricity therefrom) exist. More specifically, geothermal heat pump (GHP) systems and enhanced geothermal systems (EGS) are well known systems in the prior art for recovering energy from the Earth. In GHP systems, geothermal heat from the Earth is used to heat a fluid, such as water, which is then used for heating and cooling. The fluid, usually water, is actually heated to a point where it is converted into steam in a process called flash steam conversion, which is then used to generate electricity. These systems use existing or man-made water reservoirs to carry the heat from deep wells to the surface.

Geothermal energy is present everywhere beneath the Earth's surface. In general, the temperature of the Earth increases with increasing depth, from 400° Fahrenheit (F) to 1800° F. at the base of the Earth's crust to an estimated temperature of 6300° F. to 8100° F. at the center of the Earth. In a conventional geothermal system, such as for example and enhanced geothermal system (EGS), water is pumped into a well using a pump and piping system. The water then travels over hot rock to a production well and the hot, dirty water is transferred to the surface to generate electricity. In some situations, a phase change is involved such that the water may actually be heated to the point where it is converted into steam. The steam then travels to the surface up and out of the well. When it reaches the surface, the steam is used to power a thermal engine (electric turbine and generator) which converts the thermal energy from steam into electricity whereby the steam cools and is returned to the liquid phase as water for reuse deep in the piping system. This type of conventional geothermal system can be highly inefficient in very deep wells because of the need for large quantities of water are very limited. Furthermore, these water-based systems often fail due to a lack of permeability of hot rock within the Earth, as water injected into the well never reaches the production well that retrieves the water.

In either geothermal or solar heating systems, energy concentrators assist in maximizing collection of thermal energy. In accomplishing concentration of thermal energy, there is often more heat produced than is immediately usable thereby creating a need for storage of heat. In the instance of solar heating systems, the need for storage of heat is readily apparent in that sunshine is not a constant. Still further, thermal energy concentration and storage may also be found in any industrial process whereby heat is an intended or waste byproduct. In all of these thermal systems, water as a heat transfer medium cannot offer maximum transfer and storage characteristics. Accordingly, alternatives exist in terms of phase change material (PCM). Such PCM are materials that use phase changes (e.g., solidify, liquefy, evaporate, or condense) to absorb or release large amounts of latent heat at relatively constant temperature. Suitable materials include paraffin, salt hydrates, or water-based solutions. Regardless of the type of materials, such PCM leverage the natural property of latent heat to help maintain a temperature for extended periods of time.

Several solutions exist for PCM based thermal systems including “PureTemp PCM” from ENTROPY SOLUTIONS INC. of Plymouth, Minn. “PureTemp PCM” uses small encapsulated PCMs to store heat at constant temperature and deal with the low thermal conductivity of the PCMs. However, while encapsulation works for small volume PCMs it cannot store large volumes of heat, nor can it do so efficiently at a temperature above 300° F.

Another approach based on the use of molten salt that does not incorporate the use of PCMs is the thermal storage systems included in the “Solar Power Towers” from SolarReserve of Santa Monica, Calif. that provide power plants configured to capture and focus the sun's thermal energy with heliostats. A tower resides in the center of a heliostat field. The heliostats focus concentrated sunlight on a receiver which sits on top of the tower. Within the receiver, the concentrated sunlight heats molten salt to over 1000° F. The heated molten salt then flows into a thermal storage tank where it is stored and eventually pumped to a steam generator where the steam drives a standard turbine to generate electricity. The molten salt storage loop enables the plant to generate electricity regardless of sunshine. Although such molten salt storage technology operates at medium temperatures to extract sensible heat, this solution requires two tanks, extensive piping and expensive external heat exchangers and does not take advantage of the concentrated energy storage a PCM solution offers. It also has an upper limit in size where it will become economically unfeasible.

The Institute of Technical Thermodynamics, German Aerospace Center (DLR) in a paper on “ADVANCED HIGH TEMPERATURE LATENT HEAT STORAGE SYSTEM—DESIGN AND TEST RESULTS” states: “Different options have been investigated to overcome the limitation resulting from the low thermal conductivity of the storage material. The sandwich concept has been identified as the most promising option to realize cost-effective latent heat storage systems. Here, fins enhance the heat transfer within the storage material. The heat transfer area is increased by mounting the fins vertically to the axis of the tubes. The characteristic height of these fins exceeds the dimensions which are commercially available as finned tubes. Decisive for the successful implementation of this approach is the selection of the fin material.” This approach is a batch approach and is not continuous as once the PCM has frozen on the fin surface the PCM insulates the fin and has to be melted by a heat source. This approach is also inefficient in terms of heat flux extracted.

In the DLR paper, a fin heat exchanger is described therein as the state of the art and, although not implemented in the field, can be used in reverse mode to extract heat from the HCF and transfer it to a molten salt which is stored in a large storage tank. This heat exchanger however does not incorporate phase change on both the input and output side of the heat exchanger. Its input uses sensible heat derived from the solar HTF (typically high pressure steam or hot oil) and its output uses phase change to heat the molten salt which is then pumped to a large storage tank. That prior art approach decreases the efficiency of the heat transfer process as phase change is only used on one side of the heat exchanger. Furthermore, common technologies used in the field of solar energy collection, conversion and storage are shell and tube heat exchangers which collect thermal energy from high pressure steam or high temperature oil and transfer sensible heat to a molten salt, no phase change is employed. Improvement over such heat exchangers as implemented in the field of solar energy collection, conversion and storage is therefore needed.

It is, therefore, desirable to provide a thermal energy storage solution that overcomes the undesirable effects of the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at least one disadvantage of previous thermal energy storage solutions.

The present invention provides storage and recovery of large amounts of thermal energy for many days using phase change, for use in generating electricity, steam or for other industrial processes. Moreover, the present invention provides the ability to continuously extract medium temperature heat (100° to 600° Celsius (C.) from a molten phase change storage material with low thermal conductivity while overcoming PCM tendencies to solidify and thereby insulate the surface which extracts the heat.

The present invention does not utilize encapsulation used in small volume PCMs, but rather uses an alternative approach to eliminate the problems associated with low thermal conductivity storage material freezing on heat transfer surfaces.

The present invention uses molten PCM, which freezes over a temperature range that is bounded by its liquidus and solidus states, to extract large quantities of medium temperature heat efficiently while maintaining a temperature within the freezing range of the PCM storage medium.

The present invention provides that the temperature of the thermal storage medium remains at an almost constant temperature (bounded by its liquidus and solidus states) when extracting latent heat energy and only requiring a single tank and does not suffer the wide temperature swings that occur when extracting sensible heat energy from traditional two tank heat storage mediums which store sensible heat.

The present invention provides for a reduction of the storage footprint and capital costs typically by 75% by utilizing a single tank, eliminating need for hot/cold cycling which requires two tanks and more storage material which can be as much as four times the storage volume.

The present invention significantly extends the time period over which large amounts of heat energy can be withdrawn from storage as a result of the reduced storage size required.

In a first aspect, there is provided a method of thermal energy storage and extraction for large systems using phase change materials with low thermal conductivity, the method including: heating a phase change material; transferring the phase change material to a storage tank having a heat exchange drum and concentric scraper mechanism; and maintaining fairly constant heat transfer at a surface of the heat exchange drum via the scraper mechanism.

In a further aspect, there is provided an apparatus for thermal energy storage and extraction within large systems using phase change materials with low thermal conductivity, the apparatus including: a heat transfer loop for heating a phase change material; a heat storage loop for transferring the phase change material to a storage tank having a heat exchange drum and concentric scraper mechanism; and a working loop for maintaining constant heat transfer to working fluid via a surface of the heat exchange drum by way of the scraper mechanism.

In a further aspect, there is provided a system for thermal energy storage and extraction using phase change materials with low thermal conductivity, the system including: a heat source; a heat transfer loop for heating a phase change material with heat from the heat source; a heat storage loop for transferring the phase change material to a storage tank having a heat exchange drum and concentric scraper mechanism; and a working loop for maintaining constant heat transfer to working fluid of a power plant via a surface of the heat exchange drum by way of the scraper mechanism.

In a further aspect, there is provided an method of thermal energy extraction and storage, the method including: placing a molten phase change material in a thermal storage tank; at least partly submerging a first side of a heat transfer surface within the molten phase change material; moving heat transfer fluid across a second side of the heat transfer surface such that heat from the molten phase change material transfers from the molten phase change material to the heat transfer fluid; facilitating constant heat transfer from the molten phase change material to the heat transfer fluid by using a scraper mechanism for removal of solidified phase change material from the first side of the heat transfer surface.

In a further aspect, there is provided an apparatus for thermal energy extraction and storage, the apparatus including: a thermal storage tank for retaining a phase change material in a heated state; a heat exchanger at least partly submerged within the phase change material, the heat exchanger including a first heat transfer surface and a second heat transfer surface, the phase change material in contact with the first heat transfer surface; a heat transfer fluid in contact with the second heat transfer surface and arranged such that heat is transferred from the phase change material to the heat transfer fluid; and a scraper mechanism for removal of the phase change material from the first heat transfer surface formed via solidification of the phase change material upon the first heat transfer surface.

In still a further aspect, there is provided an apparatus for dual stage thermal energy extraction and storage, the apparatus including: a large thermal storage tank (LTST) for retaining a phase change material in a heated state; a decoupled thermal storage extractor (DTSE) for receiving the phase change material from the LTST; a latent heat to latent heat extractor (LHTLHE) for receiving heat collection fluid (HCF) in a vaporized state and for receiving the phase change material from the LTST and selectively from the DTSE, the LHTLHE having heat exchanger coils through which the phase change material flows and exits to the LTST, the heat exchanger coils configured for exposure to the HCF to enable heat transfer between the phase change material and the HCF.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

FIG. 1 is an illustration of a first preferred embodiment of the present invention as utilized with a solar linear Fresnel reflector using heat pipes.

FIG. 2 is an illustration of another embodiment of the present invention as utilized with a solar trough or linear Fresnel reflector system.

FIG. 3 is an illustration of the storage tank with heat exchanger in accordance with the present invention.

FIG. 4 is an illustration of the inner and outer drum of the heat exchanger in accordance with the present invention.

FIG. 5 is an exploded view of the heat exchanger as shown in FIG. 4 with the inner and outer drum separated for purposes of illustration.

FIGS. 6 and 7 illustrate an alternative embodiment similar to that shown in FIG. 3, but having a scraping mechanism formed by bars or wires.

FIG. 8 illustrates a further alternative embodiment having a scraping mechanism formed by an auger arranged internal to a tubular heat exchanger.

FIG. 9 illustrates yet another alternative embodiment having multiple scraping mechanisms formed by rotating blades arranged external to plate-like heat exchangers.

FIGS. 10, 11, and 12 illustrates yet still another embodiment having a decoupled thermal storage extractor with a large storage tank and related latent heat to latent heat extractor.

DETAILED DESCRIPTION

Generally, the present invention is a thermal storage extractor (TSE) that provides heat extraction and storage of medium temperature thermal energy within large systems using phase change from phase change materials with low thermal conductivity.

The TSE is generally described herein as a thermal storage tank environment containing a thermally non-conductive molten PCM which resides on the outside of a heat transfer surface and a heat transfer fluid (HTF) flowing on the inside of the heat transfer surface at a lower temperature than the PCM. This invention mechanically removes the PCM solids formed as a result of the cooling of the heat transfer surface due to the transfer of heat from the PCM to the HTF through the heat transfer surface walls. Removal of PCM solids occurs by using various scraping methods and configurations which cause the solids not only to be removed but to fall to the bottom of a tank where they are either re-heated, collected, or removed from the tank. After removal of the solids from the heat transfer surface, the surface is thereby refreshed allowing for the further transfer of heat from the molten PCM through the heat transfer surface walls to HTF until more solids form on the heat transfer surface to be removed by scraping.

The present invention uses a phase change material (two phase material-liquid, solid) by continuously removing thermal resistance that comes from the phase change material allowing the system to operate at a much higher rate of efficiency. The inventive heat exchanger is advantageously provided inside the storage tank thereby reducing heat losses, capital costs and space requirements compared to existing thermal storage systems. This enables need for only one storage tank such that three major components have been collapsed into a single unit. Moreover, the present invention facilitates the temperature of the storage medium remaining at a relatively constant temperature when extracting thermal energy and does not suffer the wide temperature swings that occur when extracting thermal energy from sensible traditional heat storage mediums.

The present invention is able to use a wide variety of low cost high latent heat PCMs to continuously extract latent heat at a fraction of the cost of existing large thermal storage systems.

The present invention is able to continuously extract latent heat from a PCM at high heat flux rates. This is accomplished by removing thermal resistance that comes from the PCM solidifying upon and insulating the heat transfer surface, allowing the system to operate at a much higher rate of efficiency. The temperature of the storage medium operates between the liquidus and solidus temperatures when extracting thermal energy and does not suffer the wide temperature swings that occur when extracting thermal energy from sensible traditional heat storage mediums.

This invention is a device to efficiently extract and optionally store large amounts of medium temperature heat energy from a thermal storage material using latent heat of fusion phase change. The inventive TSE includes a storage tank containing molten PCM with a submerged or partially submerged heat exchanger with horizontal or vertical orientation. The heat exchanger can take many forms and various implementations are shown and described herein as they perform the same function that is, remove solids from the majority of their heat exchange surface using scrapers. Four possible mechanical scraper embodiments of the heat exchangers are described in more detail hereinbelow. However, it should be readily apparent that removal of the PCM solids from the heat transfer surface may also be achieved by using ultrasound in conjunction with an insert surfactant. This approach is also considered within the scope of the present invention.

With reference to FIG. 1, the present invention 100 is a device to efficiently extract and optionally store large amounts of medium temperature heat energy from a thermal storage material using latent heat of fusion phase change. Here, heat flux Q_(in) is provided from heat concentrated from a collection system. The collection system may be of any known type such as, but not limited to, solar arrays, solar linear Fresnel reflector arrangements, solar troughs, solar towers or any suitable solar collection system. Likewise, the collection system may also be any heat generation source aside from solar including, but not limited to, heat derived with or without concentration mechanisms from industrial machinery or any heat emitting device or machine. In this regard, the heat flux Q_(in) may be provided in any available medium which is either an open or closed loop which provides the heated medium to a heat pipe 107 and, after heat transfer to the heat pipe 107, returns the cooled medium to the heat generation source.

The heat pipe 107 may be of any suitable configuration. One such suitable configuration is shown by way of a heat pipe disclosed in U.S. Pat. No. 7,115,227 issued on 03 Oct. 2006 to Mucciardi et al., and herein incorporated by reference. In terms of the Mucciardi et al. example, such a heat pipe would include an assembly under vacuum or pressure and having a liquid working substance charged therein, including generally an evaporator adapted to evaporate the working substance and a condenser. The heat exchanging condenser is in fluid flow communication with the evaporator. The condenser is adapted to condense evaporated working substance received from the evaporator and has a reservoir, located at a higher elevation than the evaporator, for collecting liquid working fluid therein. A discrete, impermeable liquid return passage permits the flow, by gravity, of the liquid working substance from the reservoir to the evaporator. The liquid return passage extends through the evaporator and terminates near the closed leading end thereof, and is fitted with a vent line that diverts ascending vapor to the top of the condenser. A flow modifier is positioned within the evaporator, causing swirling working fluid flow in the evaporator, whereby the flow modifier ensures that un-vaporized liquid entrained with evaporated working substance is propelled against inner surfaces of the evaporator by centrifugal force to ensure liquid coverage of the inner surfaces, thereby delaying onset of film boiling. The vapor/liquid loop (i.e., VAPOR and LIQUID RETURN as shown) between the heat pipe 107 and condenser/heat exchanger 106 is a closed loop providing transfer of heat flux Q_(in) to a phase change material (PCM) as shown in FIG. 1. This effectively segregates heat collection 101 from heat transfer to storage 102.

As further shown in FIG. 1, heat transferred from the heat flux Q_(in), to the PCM provides a closed PCM loop 102 a, 102 b. The PCM loop 102 a, 102 b involves heated PCM being routed into a storage tank 105 and subsequently routed out as a PCM slurry to again be heated in the condenser/heat exchanger 106 discussed above. One aspect of the present invention is that the storage tank 105 contains hot phase change liquid material with integrated heat exchanger drums 104. The heat exchanger drums 104 are shown in more detail in FIGS. 3 through 5 and are submerged in the hot phase change liquid and the entire surface of the storage tank is covered by the drums except for gaps to allow space for scrapers or other material removal devices. The purpose of the scrapers is to remove surface build-up of solidified and solidifying PCM on the heat exchanger drum outer surface. In doing so, continuous heat transfer from the PCM to the working fluid (i.e., Q_(out)) is enabled from the PCM during phase change. The working fluid is typically a loop 103 a, 103 b to a power plant 103 for electricity or steam generation for industrial purposes.

In terms of FIG. 2, as similar system 200 is shown with identical heat collection 201 and heat transfer to storage 202 operation and functional elements provided except that a simple heat exchanger 206 with input of hot oil or pressurized steam from heat pipes 207 again via a closed loop (i.e., HOT FLUID and FLUID RETURN) is utilized to heat the phase change material via PCM loop 202 a, 202 b. The PCM storage liquid or slurry to be heated at 202 b is provided to the heat exchanger 206 and returned to the at 202 a to the storage tank 105 whereby the heat exchanger 104 extracts via working fluid at 103 a the heat flux Q_(out) for use in a power plant 103 before return of the working fluid at 103 b.

With further regard to FIG. 3, a storage arrangement 300 is shown where each drum has an intake 311 and exhaust 303 pipe at its center, which extends outside the storage tank 301 on opposite sides. As mentioned, the working fluid to be used for electricity generation or other application, is pumped at a temperature lower than that of the hot phase change liquid, flowing from 310 to 304 as shown through the inside of the drum entering through the intake and exiting from the exhaust side of the drum for use in its electricity or steam generation application. The working fluid extracts heat from the hot phase change liquid through the walls of the drum. This hot phase change liquid is a molten PCM held at a working level 302 in the tank 301. The working fluid in the heat exchange drums is chosen based on its subsequent use. For some applications, it may be a liquid that is heated in the drum. An example could be oil. In other applications, it may be pressurized liquid (e.g., water) which is boiled in the drums to produce a high pressure vapor (e.g., steam). The discharge is then fed to, for example, a Rankine cycle plant to generate electricity. If the discharge is a high pressure vapor, it can be fed directly to a turbine. If it is a hot liquid (e.g., oil), it can be used to create a high pressure vapor for the turbine from a variety of liquid feeds such as organics, water, etc.

The PCM solution includes a binary or multi-component system. While a single component system may also be used, it is the preferred embodiment to use a PCM solution that freezes over a temperature range and as such is characterized by varying degrees of solid fractions in the solidifying layer which makes its removal easier. The multi component combinations of compounds may consist of, but are combinations of compounds of, but not restricted to, a material selected from the group consisting of Potassium Nitrate, Potassium Nitrite, Potassium Hydroxide, Potassium Carbonate, Potassium Chloride, Sodium Hydroxide, Salt ceramics (NaCo3−BaCo3/MgO) Sodium Nitrate, Sodium Nitrite, Sodium Hydroxide, Sodium Carbonate, Sodium Chloride, Zinc Chloride, Lithium Nitrate, Lithium Nitrite, Lithium Chloride, Magnesium Chloride, Nitrate salts, Nitrite salts, Carbonate salts, Calcium Nitrate, Calcium Nitrite, Pentaerythritol. The PCM material may also be formed by Aluminum Silicon alloy.

Still further, it should be understood that the phase change material in the tank is chosen such that it comprises a solution which has a freezing range (i.e., liquidus and solidus temperature) at temperatures required by the application (electricity generation or other). The solution may be binary or multi-component. It should therefore be readily apparent that many different PCM mixtures may be used without straying from the intended scope of the present invention. Suitable salt combinations for phase change materials can be a mixture based on the desired operating temperature and also selected in terms of the ratio of the individual compounds/components to achieve ideal maximum and minimum operating temperature points of the slurry. Moreover, any combination of compounds may be selected such that their molar composition (i.e., mixture) will operate near, but not at, the eutectic temperature point on the corresponding binary or tertiary phase diagram. This ensures that above the desired maximum operating temperature the mixture will be a liquid and below the minimum desired operating temperature the mixture will be a solid, whereas in between the maximum and minimum operating temperatures it will be a slurry formed of a mixture of solid and liquid.

With further reference to FIG. 3, PCM flow is shown from input 308 of molten PCM to output 305 of molten PCM with some PCM solids. The transfer of heat energy from the hot phase change liquid surrounding the outside wall of the drum, to the working fluid inside the drum, causes the hot phase change liquid with low thermal conductivity to start solidifying on the outer surface of the drum wall, thereby insulating the drum from absorbing more heat. As shown, hot molten PCM rises in the tank while PCM solids fall to the bottom 306. Because the phase change storage material has a freezing range, the solidification on the outer surface of the drum should be characterized by a ‘mushy’ region of slurry (i.e. mixture of liquid and solid) which is easily removed. One aspect of the present invention is to remove the solidified and mushy phase change material that insulates the drum, from the outer wall of the drum, at a constant rate by using a scraper blade 307 rotating via motor 309 around the outer wall of the drum, thereby allowing the heat transfer to the working fluid inside the drum to continue at a relatively fast rate. The outer scraper mechanism 312 with blade 307 is concentric with the inner heat exchanger drum.

It is a preferred embodiment of this invention that the frozen layer on the outside of a drum be easily removed by having it exist as a mushy material which is amenable to removal by a scraping or other suitable mechanical device. It is also a preferred embodiment that the drum remains stationary with the scraper rotating around it, so as to simplify the construction and operation of the drum units. In one possible implementation, the scraper is rotated by an external motor and chain attached to its pipe along its center axis and which extends outside the storage tank but around the intake and exhaust pipe of the stationary drum. However in other embodiments within the scope of the present invention, the drum may rotate around a fixed scraping device.

Although a particular embodiment of scraper is shown and described, it should be readily apparent that more than one scraper device configuration is possible and the scraping device may be a straight blade the length of the drum rotating around the drum. Moreover, any potential methods for renewing the heat transfer surface may be provided without straying from the intended scope of the present invention. Such variations may include mechanical separation methods such as, but not limited to: 1) flat plates with wiper arms (windshield wiper like devices); 2) turning around drum with stationary scrapers; 3) netting, grating or tightened wire that rotates closely around the surface of the drum or is stationary with a moving drum; or 4) using rollers to crush the frozen material off of the drum. As well, such variations may include mechanical, chemical or other separation methods such as, but not limited to: 1) ultrasonic bursts that remove the solid from the heat transfer surface; 2) using jets of cold slurry to separate the frozen material off of a drum or plate; 3) using jets of inert liquid to separate the frozen material off of a drum or plate; or 4) treating the heat transfer surface with a non stick coating.

In terms of a spiral scraper blade described in more detail herein, such configuration may be one preferred method of scraping the drum. The spiral scraper blade appears like a cork screw around the outside of the drum with a sharp blade on its inner side which scrapes the surface of the drum. The spiral scraper blade may have a height above the drum sufficient to displace and move hot phase change liquid away from the surface of the drum as it rotates. The spiral scraper blade's inner side may touch the surface of the drum provided that this contact does not affect its rotational ability, or it may be a distance of up to ⅛″ above the surface of the drum.

As the scraper operates, the scraped phase change material is ideally in the mushy state (e.g., a ‘slurry’), but is denser than the hot phase change liquid, and falls to the bottom of the storage tank. There, the phase change material is reheated and liquefied by incoming hot phase change liquid heated from a heating source such as a concentrated solar collector as illustrated in embodiment #1 (FIG. 1). At the bottom of the tank and on the opposite side from the arriving hot phase change liquid from the heating source, phase change liquid (and/or slurry) is pumped out of the storage tank to its heating source such as a concentrated solar collector where its temperature is raised to a temperature higher than the storage tank's before returning to the bottom of the tank to convert solid phase change material to liquid. The cycle of exhausting phase change liquid heating and returning it, is contained in a closed loop.

The construction of the storage tank is stainless or carbon steel or other suitable material capable of storing medium temperature molten PCM. To enable the efficient transfer of heat from the hot phase change material surrounding the main drum (the outer drum) to the working fluid inside, a drum of lesser diameter (the inner drum) is positioned inside the outer drum so that the layer of working fluid flowing between them that is to be heated is thin, thereby promoting faster heat transfer. The difference in diameter between the outer and inner drums will vary depending on, the planned use of the working fluid and is a function of the required heat extraction rate. As can be seen by way of FIGS. 4 and 5, the inner drum contains 408 circular flow guides 401 positioned in the gap between the inner 404 and outer 405 drums. The flow guides 401 force the working fluid to swirl around the inner drum 404 from point of entry 406 to point of exit 403 via respective channels 407 and 402, pushing the heavier and colder working fluid to the inner surface of the outer drum. Further alternative embodiments of the mechanical removal of solidified PCM from the heat exchange surface are shown and described later herein with regard to FIGS. 6, 7, 8, and 9.

With specific reference to FIGS. 6 and 7, a respective side view 600 and end view 700 of an alternative embodiment of the scraper mechanism are shown which is similar to that shown in FIGS. 4 and 5. In contrast to FIGS. 4 and 5, the embodiment of FIGS. 6 and 7 include scrapers 610 in the form of either aircraft wire or metal bars. Such aircraft wires may include braided strands of stainless steel or any similarly durable material. The use of scrapers in the form of wires which are inherently flexible allows for positive contact of the wire scraper with the outer surface of the drum which forms the heat exchange surface. Similar to the arrangement shown in FIGS. 4 and 5, the heat transfer fluid (HTF) passes in flow 613 between the inner 606 and outer 607 drums. Here, the inner 606 and outer 607 drums along with the rotating scrapers 610 (either wire or bar type) are submerged in the molten PCM 601 within a tank 603 that holds all aforementioned elements.

As such molten PCM flows from an inlet 608 to an outlet 605, the HTF flows from an inlet (at 611) to an outlet (at 602) through the space 701 between the inner 606 and outer 607 drum. In effect, the inner and outer drum form a tubular cylinder with which cylinder's walls is the aforementioned space. As the HTF flows through this space 701, heat from the molten PCM in which the drums are submerged thereby transmits into the HTF for subsequent external use in electricity or steam generation. As more heat is transferred, the molten PCM will cool near and collect upon the outer drum surface.

However, each pass of the rotating scrapers 610 (either wire or bar type) will remove such collection of cooled PCM from the drum surface so as to refresh and renew the ability of heat transfer from the outer drum surface through to the HTF. Accordingly, hot molten PCM will enter at 608 while molten PCM with PCM solids will exit at 605.

As shown in the embodiment of FIGS. 6 and 7, rotation of the rotating scrapers 610 (either wire or bar type) may be accomplished via an external motor (not shown) with a driveshaft 612 having a worm gear drive 609. The worm gear would then drive a geared disc to effect rotational movement upon the scrapers which are mounted on rotational bearings 604. Such mechanics of worm gears and bearing structure are well within the skill of those in the mechanical arts and are not further described herein.

During flow of molten PCM, flow of HTF, and related removal of solidified PCM by the scrapers, it should also be understood that PCM solids will fall by normal gravity to the bottom of the tank. As can be seen in FIG. 7, the drums are arranged in the warmer top half of the tank as heated molten PCM rises. In the course of molten PCM flow from the inlet to the outlet, such solids will be also moved out of the tank. Subsequent re-heating of the PCM from a mixed slurry state to a fully molten state will then be allowed to occur via solar heating in a manner as previously discussed. The fully molten PCM will then be returned to the tank for continuous heating of the HTF and therefore continuous electrical or steam generation by way of the HTF. It should be readily apparent that any heat driven elements such as, but not limited to, steam turbines, Stirling engines, heating fins, or the like may use the heat transferred to the exiting HTF without straying from the intended scope of the present invention.

Accordingly, the first two embodiments of the present invention use drums as the heat exchangers with their outer surfaces acting as their heat exchange surfaces, in one case scraped by an auger and in the other scraped by airplane wires or metal bars.

Within the third embodiment 800 of the present invention, there is shown by FIG. 8 a further heat exchanger within a tank 802 of molten PCM 812 itself flowing from an inlet 810 to an outlet 805. Here, the HTF flows from an inlet 803 to an outlet 808 through the space 801 between the inner 806 and outer 807 drum where the inner and outer drum form a hollow, tubular cylinder with which cylinder's walls is the aforementioned space 801. As the HTF flows through this space 801, heat from the molten PCM in which the drums are submerged thereby transmits via the inner drum surface into the HTF for subsequent external use in electricity or steam generation. As more heat is transferred, the molten PCM will cool near and collect upon the inner drum surface.

However, each pass of a rotating auger type scraper 804 will remove such collection of cooled PCM from the drum surface so as to refresh and renew the ability of heat transfer from the outer drum surface through to the HTF. As before, any solidified PCM will fall to the bottom of the tank where the flow of molten PCM will mix therewith and serve to remove the molten/solid slurry of PCM to the outlet for re-heating. In this embodiment, a chain or belt 811 driven by an external motor (not shown) may be used to rotate the auger 804 which itself is rotatably mounted on an axle or pipe 809 to rotate within the stationary heat exchanger.

Yet another embodiment 900 of the present invention is shown in FIG. 9. Here, the tubular cylinder structure formed previously by drums of the prior embodiments is replaced with flattened and hollowed disc-like plates 908. Each plate 908 is circular in shape with a central aperture though which a rotating axle or pipe 901 extends and to which scrapers 902, 906 are attached. In this manner, the HTF flows (via inlet 910 and outlet 905) through the hollowed interior of each disc-like plate 908. The outer surfaces of these plates act as the heat exchange surfaces. Similar to before, any build-up of solidified PCM will be removed by the scrapers 902, 906. In this specific embodiment, the scrapers 902, 906 are configured as multiple blades that look somewhat like the propellers of the engine of a turboprop airplane. These blades are placed in such a manner so as to scrape both the top (via 902) and bottom (via 906) surfaces of each plate. As the blades sweep over the plate surfaces, any solidified PCM will be moved off the plate surfaces and thereby drop to the bottom of the tank 903 where the flow of molten PCM will move the mixed slurry to the outlet 907 for subsequent re-heating and return to the molten PCM inlet 909 as in previous embodiments to maintain a molten PCM level 904. It should be readily apparent that this configuration using plates advantageously increased the surface area available for heat transfer.

In each of the above-described embodiments of the present invention, the heat exchangers are submerged in molten PCM contained in a thermal storage tank. The heat exchangers all have an intake and exhaust pipe for passage of the HTF through them. These pipes extend past the walls of the thermal storage tank containing the molten PCM and heat exchanger enabling the HTF to be injected and recovered from outside the thermal storage tank. The HTF to be used for electricity generation or other application is pumped at a temperature lower than that of the molten PCM, through the heat exchanger entering through the intake pipe and exiting from the exhaust pipe. The HTF extracts heat from the molten PCM through the walls of the heat exchanger. The type of HTF used for this task is chosen based on its subsequent use. For some applications, it may be a liquid that is heated in the heat exchanger. An example could be an oil. In other applications, it may be pressurized liquid (e.g., water) which is boiled in the heat exchangers to produce a high pressure vapor (e.g., steam). The exhaust HTF is then fed to, for example, a Rankine cycle plant to generate electricity. If the discharge is a high pressure vapor, it can be fed directly to a turbine. If it is a hot liquid (e.g., oil), it can be used to create a high pressure vapor for the turbine from a variety of liquid feeds such as organics, water, etc.

The PCM in the thermal storage tank is chosen such that it includes a molten solution typically with high latent energy, which has a freezing range (i.e., liquidus and solidus temperature) at temperatures required by the application (electricity generation or other). The PCM solution comprises a binary or multi-component system. While a single component system may also be used, it is the preferred embodiment to use a PCM solution that freezes over a temperature range and as such is characterized by varying degrees of solid fractions in the solidifying layer which makes its removal easier. The transfer of heat energy from the molten PCM surrounding the outside wall of the heat exchanger, to the HTF inside the heat exchanger, causes the molten PCM with low thermal conductivity to start solidifying on the outer surface of the heat exchanger wall, thereby insulating the heat exchanger from absorbing more heat. Because the phase change storage material has a freezing range, the solidification on the outer surface of the heat exchanger should be characterized by a ‘mushy’ slurry region (i.e., a mixture of liquid and solid) which is easily removed.

One of the goals of this invention is to remove the solidified and mushy PCM that insulates the heat exchanger, from the outer wall of the heat exchanger, at a constant rate by using a scraper blade rotating around or on the outer or inner wall (depending on the heat exchanger configuration as previously described above) of the heat exchanger, thereby allowing the heat transfer from the PCM to the HTF through the heat exchanger walls to continue at a relatively fast rate. It is a preferred embodiment of this invention that the solidified layer on the outside of a heat exchanger be easily removed by having it exist as a mushy material which is amenable to removal by a scraping device. It is also a preferred embodiment that the heat exchanger surface remain stationary with the scraper blade rotating around or on the outer or inner wall (depending on the heat exchanger configuration), so as to simplify the construction and operation of the heat exchanger units.

The scraper is rotated by an external motor and chain (“drive system”) which rotates a drive shaft that either directly rotates the scrapers or does so through use of a gear and chain system as shown in the illustrations of all embodiments. One method of rotating the scraper is to attach the drive system to a drive shaft along its center axis and which extends outside the storage tank but around the intake and exhaust pipe of the stationary heat exchanger. However in other embodiments the heat exchanger may rotate around a fixed scrapping device. As well, more than one scraper device configuration is possible and the scraping device may be a straight blade the length of the heat exchanger rotating around the heat exchanger. A spiral scraper blade may be the most practical method of scraping the heat exchanger as shown in the illustration of the first and second embodiments. As noted above with regard to the first embodiment, a spiral scraper blade looks like a cork screw around the outside of the heat exchanger with a sharp blade on its inner side which scrapes the surface of the heat exchanger. The spiral scraper blade may have a height above the heat exchanger sufficient to displace and move molten PCM away from the surface of the heat exchanger as it rotates. The spiral scraper blade's inner side may touch the surface of the heat exchanger provided that this contact does not affect its rotational ability.

In operation, the scraped PCM is ideally in the mushy slurry state and is denser than the molten PCM so that it falls to the bottom of the thermal storage tank. There, it may be reheated and liquefied by merely incoming molten PCM heated from a heating source such as a concentrated solar collector as illustrated in FIG. 1, but may also be removed from the thermal storage tank for reheating and liquefied elsewhere. In the instance where the denser scraped PCM falls to the bottom of the tank and is reheated elsewhere, it falls on the opposite side from the arriving molten PCM. The mixed slurry of PCM is then pumped out of the storage tank to its heating source such as a concentrated solar collector where its temperature is raised to a temperature higher than the storage tank's before returning to the bottom of the tank. The cycle of exhausting PCM slurry, heating and returning it, is contained in a closed loop.

As mentioned, the construction of the storage device is stainless or carbon steel or other suitable material capable of storing medium temperature molten PCM. In the instance of Chlorides and other corrosive PCMs, suitable materials will have to be selected for the thermal storage tank and heat exchanger that enable it to operate at medium temperatures for many years.

In operation, the present invention effectuates the efficient and continuous storage and discharge of heat at high fluxes at relatively constant temperatures. The typical industrial uses include:

-   -   Use for the continuous generation of electricity from solar         thermal heat concentration systems such as solar trough, solar         tower, Linear Fresnel Reflectors as well as from other heat         concentration or collection systems.     -   Use for reducing the levelized cost of electricity for solar         thermal heat concentration systems such as solar trough, solar         tower, Linear Fresnel Reflectors to near or at grid parity         rates. This is achieved by significantly increasing the capacity         of the solar thermal plant to produce electricity around the         clock.     -   Use of steam for water and plant heating.     -   Use in industrial manufacturing processes.     -   Use in generating steam for utility use.     -   Use in storage and reuse of industrial waste heat for various         applications.     -   Use in storage for other thermal power generating technologies         (such as coal or natural gas) enabling their base load power         stations to provide peak load power.

In yet a further embodiment of the present invention, there is shown in FIGS. 10, 11, and 12 an improvement upon extraction from large thermal storage systems using phase change materials and latent heat exchangers utilizing the above-referenced and described thermal storage extractors (TSEs). Specifically, the improvement includes thermal heat extraction from, and charging of, a large thermal storage tank (LTST) containing thousands of megawatt hours of thermal energy, using both the phase change of heat collection fluid (HCF) and the phase change of molten phase change material (PCM) for thermal storage use in generating electricity, steam, or for other industrial processes as implemented in the field of solar energy collection, thermal storage and extraction. This dual use of phase change is termed herein as forming a latent heat to latent heat exchanger (LHTLHE) (shown and described later with specific regard to FIG. 11). In terms of the LHTLHE as implemented in the field of solar energy collection, conversion and storage, the following paragraphs of description shall refer to the process of adding thermal energy at a specific phase change temperature to a mix of molten and solid PCM contained in a LTST as “charging” the LTST and the process of extracting thermal energy from an LTST containing molten PCM at a specific temperature is herein referenced to be “extraction.”

As an overview, FIG. 12 shows an embodiment 1200 of the present invention in a generalized schematic format with a heat collection segment 1201 along with transfer and storage elements formed by LTST 1013, DTSE 1001, and via heat exchanger 1205 a subsequent useful output Q_(out) for use by power plant 1207. The LHTLHE shown as 1100 in FIG. 12 is likewise shown in more detail in FIG. 11. The LTST shown as 1013 and DTSE shown as 1001 in FIG. 12 are likewise shown in more detail in FIG. 10. It should be readily apparent that variations in mechanical implementation of each element are possible without straying from the intended scope of the present invention.

With specific regard to FIG. 10, an embodiment 1000 is shown. Here, an LTST 1013 is shown which includes a decoupled thermal storage extractor (DTSE) unit 1001. The DTSE 1001 is substantially identical to the TSE described above except that the DTSE 1001 is constructed so that the volume of the tank containing the heat exchanger is not much larger than the volume occupied by the heat exchanger. Thus, the heat exchanger 1205 in the DTSE 1001 is large enough to allow the heat exchanger to be 100% surrounded by (i.e., submerged within) molten PCM, but not large enough for the tank to act as a molten PCM thermal storage device. The heat exchanger, and thus the DTSE as a whole, is sized in accordance with the heat extraction requirement for a particular application. In other words, more heat requirements in terms of the HTF output at 1003 for a given application (e.g., power plant 1207 such as turbine) would require a larger heat exchanger and reduced heat requirements a smaller heat exchanger. HTF return from power plant 1207 completes the HTF loop at 1002. The other details regarding the DTSE will not be further described again as they correlate to the four embodiments of the TSE fully described hereinabove. Because the DTSE does not technically serve to store heat due to these sizing constraints, it effectively provides a TSE that is decoupled from the storage aspect. Hence, the name decoupled TSE or simply DTSE.

It should therefore be readily apparent that certain advantages exist using the DTSE as shown.

With particular to decoupling of the TSE and LTST and the extraction of thermal energy, by decoupling the TSE from its LTST, the TSE's size, placement, and operation becomes independent of the LTST's size, operation, and location. As well, decoupling the TSE from its LTST enables multiple TSEs to be used to extract heat from its LTST. Moreover, decoupling the TSE from its LTST enables heat to be extracted from the LIST at the same time the LTST is being charged. Still further, decoupling the TSE from its LTST enables distributed storage configurations of smaller LTSTs. Even more, decoupling the TSE from its LTST simplifies access and maintenance of the TSE, and there is no concern of heat energy distribution in the LIST as long as the TSE has access to molten PCM.

With particular regard the LHTLHE as implemented in the field of solar energy collection, the present inventive LHTLHE provides the most efficient method of transferring thermal energy collected by a solar thermal energy collection system to thermal storage where it can be extracted. This is accomplished by the transfer between two materials in three states (i.e., solid, liquid, and vapor) at each material's respective phase change temperature.

This decoupled TSE in terms of the presently discussed LTST/LHTLHE embodiment mechanically removes the PCM solids formed as a result of the cooling of the heat transfer surface due to the transfer of heat from the PCM to the HTF through the heat transfer surface walls, by using various scraping methods and configurations which cause the solids not only to be removed but to fall to the bottom of a tank. Unlike the earlier embodiments, here the solids (i.e., mixed slurry of molten PCM and solids) are pumped from the tank either directly to the LHTLHE for reheating, or optionally, to the LIST for heating at some later time. As can be seen in FIG. 10 and also by way of FIG. 12, placement of the DTSE 1001 is close to the top of the LTST 1013 where it accepts molten PCM from the top of the tank and exhausts molten and solid PCM (at 1004) back to the LTST (at 1006) or directly to the LHTLHE for immediate reheating (at 1005) via an actuated valve 1210. Moreover, a chimney pipe 1011 is provided within the PCM bath (denoted by liquid line 1012) which allows hot molten PCM to flow freely from the input source 1102 to the top of the LTST tank. The decision of whether to exhaust molten and solid PCM back to the LTST (at 1006) or directly to the LHTLHE for reheating (at 1005) is a function of the specific system design for the particular solar collection and electricity or steam generation system and whether the LHTLHE is operational.

With further regard to FIGS. 10 through 12, the charging process of a LTST will now be described. Here, the LTST 1013 can be seen with the DTSE 1001 outputting to the LHTLHE 1100 when charging the LHTLHE 1100 and to the LTST 1013 when not charging the LHTLHE 1100.

Initially, a heat collection fluid, HCF, (i.e., working fluid) is initially heated Q_(in) by a heat source (not shown) such as a solar thermal heat collection system or any number of sources. Having been mentioned earlier, such heat sources are not shown or described further here. The HCF is heated to a temperature where it vaporizes in the LHTLHE 1100, and such temperature is set and controlled by the pressure in the HCF pipe 107 by well-known thermodynamic principals via a gas trap valve 1107. The LHTLHE chamber 1101 operates at the same pressure as the HCF pipe 107. The HCF vapor is released into the top (at 1108) of the LHTLHE chamber 1101. There, the HCF vapor surrounds PCM heat exchanger coils 1106. These coils 1106 are pipes that effectively cool the HCF vapor as the pipes containing the mix of molten and solid PCM is at a lower temperature than the HCF vapor. The input 1109 illustrated in FIG. 11 is formed by the molten/solid PCM mix of loops 1005 and 1008.

Upon giving up its latent heat, the HCF vapor incurs a phase change and becomes a liquid. This liquid then falls to the bottom of the LHTLHE chamber 1101 and is pumped through the HCF liquid return 1104 back to its heat source to be vaporized. Fluid level is maintained at the working fluid liquid line 1103 as shown by means of adding (or removing) HCF via the HCF fluid charging port 1105.

The mix of molten and solid PCM input (at 1109) to the LHTLHE 1100 originates from the DTSE 1001 which has extracted latent energy (via the liquid to solid phase change) from the molten PCM (received at 1203) of the LTST 1013. The internal functioning of the DTSE 1001, having been described earlier, will not be further discussed here. The DTSE 1001 thereby exhausts a mix of molten and solid PCM (at 1004), which is pumped (during this charging scenario) through a pipe into the LHTLHE chamber. It should be readily apparent that the mix of molten and solid PCM is at a lower temperature than both the HCF vapor and the molten PCM in the LTST. As the mix of molten and solid PCM (from loops 1005 and/or 1008—or collectively shown at 1109) passes within the PCM heat exchanger coils 1106 and through the vapor in the LHTLHE chamber 1101, heat transfers to the PCM mix so as to acquire the latent heat from the HCF vapor. This latent heat transfer melts the PCM solid in the molten solid mix. Thus, the temperature of the PCM in the coils 1106 rises to one which is slightly higher than the temperature of the PCM in the LTST 1013. This molten PCM is then pumped from the LHTLHE chamber back to the LTST (at 1102) where it is stored until its latent heat is extracted by the DTSE and circulated for charging again.

It should be understood that during periods of time when the heat source (not shown) is insufficient to heat the HCF vapor, there may be a requirement to add heat to the LTST 1013 to ensure sufficiently molten PCM. In such scenario, auxiliary heater tubes 1009 are provided for heating via propane or any other suitable auxiliary fuel. Sensing and monitoring of the temperatures in the LTST 1013 may be accomplished by selectively placed heat probes or via a heat tracer wire 1007 as shown.

The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto. 

1. A method of thermal energy storage and extraction for large systems using phase change materials with low thermal conductivity, said method comprising: heating a phase change material; transferring said phase change material to a storage tank having a heat exchange drum and concentric scraper mechanism; and maintaining fairly constant heat transfer at a surface of said heat exchange drum via said scraper mechanism.
 2. The method as claimed in claim 1 wherein said scraper mechanism removes solidified phase change material from said surface of said heat exchange drum.
 3. An apparatus for thermal energy storage and extraction within large systems using phase change materials with low thermal conductivity, said apparatus comprising: a heat transfer loop for heating a phase change material; a heat storage loop for transferring said phase change material to a storage tank having a heat exchange drum and concentric scraper mechanism; and a working loop for maintaining constant heat transfer to working fluid via a surface of said heat exchange drum by way of said scraper mechanism.
 4. The apparatus as claimed in claim 3 wherein said scraper mechanism removes solidified phase change material from said surface of said heat exchange drum.
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 8. A method of thermal energy extraction and storage, said method comprising: placing a molten phase change material in a thermal storage tank; at least partly submerging a first side of a heat transfer surface within said molten phase change material; moving heat transfer fluid across a second side of said heat transfer surface such that heat from said molten phase change material transfers from said molten phase change material to said heat transfer fluid; facilitating constant heat transfer from said molten phase change material to said heat transfer fluid by using a scraper mechanism for removal of solidified phase change material from said first side of said heat transfer surface.
 9. The method as claimed in claim 8, said method further including re-heating said solidified phase change material removed from said first side of said heat transfer surface by said scraper mechanism.
 10. The method as claimed in claim 9 wherein said placing step includes said molten phase change material removed by said scraper mechanism.
 11. (canceled)
 12. The apparatus as claimed in claim 4 wherein said heat exchange drum is a tubular cylinder formed by an outer drum and an inner drum and said working fluid is located between said outer drum and inner drum.
 13. The apparatus as claimed in claim 12 wherein said outer drum forms said a heat exchange surface.
 14. The apparatus as claimed in claim 13 wherein said scraper mechanism is formed by aircraft wires configured so as to rotate about said outer drum.
 15. The apparatus as claimed in claim 13 wherein said scraper mechanism is formed by metal bars configured so as to rotate about said outer drum.
 16. The apparatus as claimed in claim 12 wherein said inner drum forms a heat exchange surface.
 17. The apparatus as claimed in claim 16 wherein said scraper mechanism is formed by an auger configured so as to rotate within said inner drum.
 18. The apparatus as claimed in claim 4 wherein said heat exchange drum includes at least one hollow disc, said a first heat transfer surface being an external surface of said disc and a second heat transfer surface being an internal surface of said disc, and said disc having an inlet and an outlet for flow of said working fluid therethrough.
 19. The apparatus as claimed in claim 18 wherein said disc includes a central aperture through which a rotating axle passes and said scraper mechanism is formed by blades mounted on said rotating axle and configured so as to rotate across said external surface of said disc.
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